Cosmic filaments have been recognised as influencing galaxy evolution, but their connection to galactic cold gas reservoirs is elusive, particularly at low redshifts. We investigate the influence of filaments on neutral hydrogen (HI) detections in galaxies at 0.02<z<0.09, using the MeerKAT International GHz Tiered Extragalactic Exploration (MIGHTEE) survey in the COSMOS and XMM-LSS fields. We measure the fraction of optical galaxies from the Dark Energy Spectroscopic Instrument (DESI) Survey detected in HI emission as a function of stellar mass, morphology, local density, and the distance to the nearest filament. We find that galaxies closer to filaments have higher stellar masses, higher local densities, and a larger early-type fraction, all of which affect the HI detection. After controlling for these dependencies, filaments have measurable effects on HI detection. Late-type galaxies in low-local density environments show reduced HI detection fractions close to filaments across all stellar mass ranges, suggesting potential gas depletion through ram pressure stripping and cosmic web detachment. Massive early-type galaxies show an enhanced HI detection fraction in filaments compared to the voids, suggesting possible gas replenishment through filamentary accretion or gas-rich mergers. Our results suggest that cosmic filaments influence galactic HI gas reservoirs through various competing processes, whose relative importance varies with stellar mass and local density.

Parallel to the transformation in radio continuum and H I observations from SKAO pathfinders and precursors over the past decade has been the development of a new generation of multi-object spectrographs that will be equally transformational in a broad range of scientific areas. For all extragalactic radio source populations, this spectroscopy is essential for providing precise redshifts, separating star-formation and AGN activity, identifying accretion modes and revealing detailed host galaxy properties. It is only with the detailed emission line and optical continuum diagnostics from spectroscopy that we can begin to link the AGN and star-formation activity revealed by the radio continuum to the kinematic and chemical histories of galaxies. Crucially, extensive spectroscopy also unlocks the full potential of H I observations by enabling statistical measures of the H I content of galaxies out to z = 1 and beyond, through spectral stacking analyses, as well as comprehensively tracing the local environment and large-scale structure to enable studies of environmental effects on the baryon cycle. We outline the scientific synergies enabled by combining SKAO continuum and H I surveys with current optical spectroscopic surveys, as well as identifying the needs and scientific potential of future spectroscopic surveys dedicated to the radio source population with both existing and planned optical facilities.

Untargeted neutral hydrogen (HI) surveys are well suited to identifying low surface brightness galaxies (LSBGs) that are gas rich, and they offer a complementary view to optically selected populations. We examined the LSBG population as identified via stellar and gaseous content using the MIGHTEE HI XMM-LSS early science data and the publicly available catalogs of optically identified LSBGs. There is currently little overlap between these datasets, with only three galaxies commonly detected. We performed surface brightness photometry of selected MIGHTEE HI detections to find 29 LSBGs, and 26 of these meet the size requirement (R_eff > 1.5 kpc) to be ultra-diffuse galaxies (UDGs). Furthermore, we extracted HI spectra at the location of all optically identified galaxies, placing upper limits on the HI-to-stellar mass ratio in these systems. While the HI-identified population overall tends toward bluer colors, the HI-identified and the optically selected samples mostly overlap in mean effective surface brightness, effective radii, and color. Although it is not straightforward to discern why the HI-identified LSBGs were missed in optical searches, this work highlights the utility of HI surveys in finding these faint systems. The HI-identified LSBGs are gas rich compared to the general HI-selected population. Furthermore, three out of four HI-selected UDGs with available kinematics show no systematic offset from the baryonic Tully-Fisher relation, although we are biased away from sources with low rotational velocities due to the low spectral resolution of the data. This work demonstrates the utility of HI observations for finding and characterizing the low surface brightness Universe.

We present a catalogue of HI sources extracted from the MIGHTEE survey data cubes covering the COSMOS field. The catalogue contains 293 sources in the redshift range of 0 . 004 < z < 0 . 093 . In addition to HI masses and velocity widths, the catalogue includes optical through near-infrared photometry and inferred stellar masses and star-formation rates. The quantity of sources in the HI catalogue acquired through untargeted source finding is greatly influenced by the source finding methods used. This study therefore also provides a well-characterized expected completeness of the detected sample of galaxies based on their properties, informing of any detection biases, inferred through a comparative study of different source finding algorithms. We have tested the performance of widely-used source finders: PYBDSF, PROFOUND, and SOFIA, along with new source finder LESHI, focusing exclusively on HI source detection rather than source characterization in the first instance. The source finders were tested by injecting a sample of simulated galaxies divided into narrow bins of mass, inclination and distance into a MeerKAT data cube. The results inform the source finding strategies for the MeerKAT International GigaHertz Tiered Extragalactic Exploration (MIGHTEE) survey, as well as upcoming SKAO surveys.

The baryonic Tully-Fisher relation (bTFR) and the radial acceleration relation (RAR) link the observed dynamics in galaxies to that expected from their baryonic mass distributions. The relations' small intrinsic scatters place strong constraints on galaxy formation models, dark matter properties and theories of modified dynamics, yet detailed measurements beyond the very local Universe remain limited. We use 130 purely HI-selected galaxies with resolved HI kinematics and baryonic mass profiles to measure the bTFR and RAR up to z≈0.09. We measure a tight RAR with an acceleration scale a_0=(1.50±0.05)×10^-10, m,s^-2 and an intrinsic scatter of 0.096±0.006 dex, consistent with local results. We fit the bTFR in the `inverse' direction, conditioning on M_ bar to mitigate HI flux-related selection effects, measuring a logarithmic slope of 0.27±0.01 (corresponding to a forward slope of 3.72±0.16), with vertical intrinsic scatter σ_⊥≈0.05 dex. Fitting the general δ-family of MOND interpolating functions to the RAR, we infer δ=4.10^+1.4_-0.68, consistent with the value required by Solar System gravitational constraints and a null Wide Binary Test. We find no significant redshift evolution in the RAR acceleration scale for our pure HI-selected sample. However, the bTFR zero-point shows an apparent evolutionary trend that is strongly dependent on the fit direction: the traditional forward fit yields an 8.7σ preference for z evolution, while for our fiducial inverse fit, this reduces to 3.4σ, within ≈2σ of the RAR evolution constraint. This suggests selection effects bias the forward fit; a careful consideration of such effects will be required in future endeavours to robustly measure the redshift evolution of dynamical scaling relations.

The relationship between the already formed stellar mass in a galaxy and the gas reservoir of neutral atomic hydrogen, is a key element in our understanding of how gas is turned into stars in galaxy haloes. In this paper, we measure the M_ HI-M_⋆ relation based on a stellar-mass selected sample at 0.25 < z < 0.5 and the MIGHTEE-HI DR1 spectral data. Using a powerful Bayesian stacking technique, for the first time we are also able to measure the underlying bivariate distribution of HI mass and stellar mass of galaxies with M_⋆ > 10^9.5 M_⊙, finding that an asymmetric underlying HI distribution is strongly preferred by our complete samples. We define the concepts of the average of the logarithmic HI mass, ⟨log_10(M_ HI)⟩, and the logarithmic average of the HI mass, log_10(⟨ M_ HI⟩), and find that the difference between ⟨log_10(M_ HI)⟩ and log_10(⟨ M_ HI⟩) can be as large as ∼0.5 dex for the preferred asymmetric HI distribution. We observe shallow slopes in the underlying M_ HI-M_⋆ scaling relations, suggesting the presence of an upper HI mass limit beyond which a galaxy can no longer retain further HI gas. From our bivariate distribution we also infer the HI mass function at this redshift and find tentative evidence for a decrease of 2-10 times in the co-moving space density of the most HI massive galaxies up to z∼ 0.5.

The relationship between the already formed stellar mass in a galaxy and the gas reservoir of neutral atomic hydrogen, is a key element in our understanding of how gas is turned into stars in galaxy haloes. In this paper, we measure the M-HI - M-star relation based on a stellar-mass selected sample at 0.25 < z < 0.5 and the MeerKAT International GHz Tiered Extragalactic Exploration-H I Data Release 1 spectral data. Using a powerful Bayesian stacking technique, for the first time we are also able to measure the underlying bivariate distribution of H I mass and stellar mass of galaxies with M-star > 10(9.5) M-circle dot, finding that an asymmetric underlying H I distribution is strongly preferred by our complete samples. We define the concepts of the average of the logarithmic H I mass, < log(10) (M-HI)>, and the logarithmic average of the H I mass, log(10) (< M-HI >), and find that the difference between < log(10) (M-HI)> and log(10) (< M-HI >) can be as large as similar to 0.5 dex for the preferred asymmetric H I distribution. We observe shallow slopes in the underlying M-HI - M-star scaling relations, suggesting the presence of an upper H I mass limit beyond which a galaxy can no longer retain further H I gas. From our bivariate distribution we also infer the H I mass function at this redshift and find tentative evidence for a decrease of 2-10 times in the comoving space density of the most H I massive galaxies up to z similar to 0.5.

The radial acceleration relation (RAR) is a fundamental relation linking baryonic and dark matter in galaxies by relating the observed acceleration derived from dynamics to the one estimated from the baryonic mass. This relation exhibits small scatter, thus providing key constraints for models of galaxy formation and evolution - allowing us to map the distribution of dark matter in galaxies - as well as models of modified dynamics. However, it has only been extensively studied in the very local Universe with largely heterogeneous samples. We present a new measurement of the RAR, utilizing a homogeneous sample of 19 H I-selected galaxies out to z=0.08. We introduce a novel approach of measuring resolved stellar masses using spectral energy distribution fitting across 10 photometric bands to determine the resolved mass-to-light ratio, which we show is essential for measuring the acceleration due to baryons in the low-acceleration regime. Our results reveal a tight RAR with a low-acceleration power-law slope of similar to 0.5, consistent with previous studies. Adopting a spatially varying mass-to-light ratio yields the tightest RAR with an intrinsic scatter of only 0.045 +/- 0.022 dex, highlighting the importance of resolved stellar mass measurements in accurately characterizing the gravitational contribution of the baryons in low-mass, gas-rich galaxies. We also find the first tentative evidence for redshift evolution in the acceleration scale, but more data will be required to confirm this. Adopting a more general MOND interpolating function, we find that our results ameliorate the tension between previous RAR analyses, the Solar System quadrupole, and wide-binary test.

We highlight the potential benefits of a synergistic use of SKAO and ESO facilities for galaxy evolution studies, focusing on the role that ESO spectroscopic surveys can play in supporting next-generation radio continuum and atomic hydrogen (HI) surveys. More specifically we illustrate the role that currently available or soon to be operational ESO multiplex spectrographs can play for three classes of projects: large/deep redshift survey campaigns, integral field unit/Atacama Large Millimeter/submillimeter Array (IFU/ALMA) surveys of selected regions of sky, and IFU/ALMA follow-ups of selected samples. We conclude with some general recommendations for an efficient joint exploitation of ESO-SKAO surveys.

Atomic hydrogen constitutes the gas reservoir from which molecular gas and star formation in galaxies emerges. However, the weakness of the line means it has been difficult to directly detect in all but the very local Universe. Here, we present results from the first search using the MeerKAT International Tiered Extragalactic Exploration (MIGHTEE) Survey for high-redshift (z>0.25) H I emission from individual galaxies. By searching for 21-cm emission centred on the position and redshift of optically selected emission-line galaxies we overcome difficulties that hinder untargeted searches. We detect 11 galaxies at z>0.25, forming the first sample of z>0.25 detections with an interferometer, with the highest redshift detection at z = 0.3841. We find they have much larger H I masses than their low-redshift H I-selected counterparts for a given stellar mass. This can be explained by the much larger cosmological volume probed at these high redshifts, and does not require any evolution of the H I mass function. We make the first-ever measurement of the baryonic Tully-Fisher relation (bTFr) with H I at z>0.25 and find consistency with the local bTFr, but with tentative evidence of a flattening in the relation at these redshifts for higher-mass objects. This may signify evolution, in line with predictions from hydrodynamic simulations, or that the molecular gas mass in these high-mass galaxies could be significant. This study paves the way for future studies of H I beyond the local Universe, using both searches targeted at known objects and via pure H I selection.

ABSTRACT The tight relationship between infrared luminosity (LTIR) and 1.4 GHz radio continuum luminosity ($L_\mathrm{1.4\, GHz}$) has proven useful for understanding star formation free from dust obscuration. Infrared emission in star-forming galaxies typically arises from recently formed, dust-enshrouded stars, whereas radio synchrotron emission is expected from subsequent supernovae. By leveraging the wealth of ancillary far-ultraviolet – far-infrared photometry from the Deep Extragalactic VIsible Legacy Survey and Galaxy and Mass Assembly surveys, combined with 1.4 GHz observations from the Meer Karoo Array Telescope International GHz Tiered Extragalactic Exploration survey and Deep Investigation of Neutral Gas Origin projects, we investigate the impact of time-scale differences between far-ultraviolet – far-infrared and radio-derived star formation rate (SFR) tracers. We examine how the spectral energy distribution (SED)-derived star formation histories (SFHs) of galaxies can be used to explain discrepancies in these SFR tracers, which are sensitive to different time-scales. Galaxies exhibiting an increasing SFH have systematically higher LTIR and SED-derived SFRs than predicted from their 1.4 GHz radio luminosity. This indicates that insufficient time has passed for subsequent supernovae-driven radio emission to accumulate. We show that backtracking the SFR(t) of galaxies along their SED-derived SFHs to a time several hundred megayears prior to their observed epoch will both linearize the SFR–$L_\mathrm{1.4\, GHz}$ relation and reduce the overall scatter. The minimum scatter in the SFR(t)–$L_\mathrm{1.4\, GHz}$ is reached at 200 – 300 Myr prior, consistent with theoretical predictions for the time-scales required to disperse the cosmic ray electrons responsible for the synchrotron emission.

The MIGHTEE survey utilizes the South African MeerKAT radio telescope to observe four extragalactic deep fields, with the aim of advancing our understanding of the formation and evolution of galaxies across cosmic time. MIGHTEE's frequency coverage encompasses the HI line to a redshift of z similar or equal to 0.58, and OH megamasers to z similar or equal to 0.9. We present the MIGHTEE-HI imaging products for the COSMOS field, using a total of 94.2 h on-target and a close-packed mosaic of 15 individual pointings. The spectral imaging covers two broad, relatively interference-free regions (960-1150 and 1290-1520 MHz) within MeerKAT's L-band, with up to 26 kHz spectral resolution (5.5 km s(-1) at z = 0). The median noise in the highest spectral resolution data is 74 mu Jy beam(-1), corresponding to a 5 sigma HI mass limit of 10(8.5) M-circle dot for a 300 km s(-1) line at z = 0.07. The mosaics cover >4 deg(2), provided at multiple angular resolution / sensitivity pairings, with an angular resolution for HI at z = 0 of 12 arcsec. We describe the spectral line processing workflow that will be the basis for future MIGHTEE-HI products, and validation of, and some early results from, the spectral imaging of the COSMOS field. We find no evidence for line emission at the position of the z = 0.376 HI line reported from the CHILES survey at a >94 per cent confidence level, placing a 3 sigma upper limit of 8.1 x 10(9) M-circle dot on MHI for this galaxy. A public data release accompanies this article.

We present a new machine learning (ML)-driven source-finding tool for next-generation radio surveys that performs fast source extraction on a range of source morphologies at large dynamic ranges with minimal parameter tuning and post-processing. The construction of the Square Kilometre Array (SKA) radio telescope will revolutionize the field of radio astronomy. However, accurate and automated source-finding techniques are required to reach SKA science goals. We have developed a novel source-finding method, ContinUNet, powered by an ML segmentation algorithm, U-Net, that has proven highly effective and efficient when tested on SKA precursor data sets. Our model was trained and tested on simulated radio continuum data from SKA Science Data Challenge 1 and proved comparable with the state-of-the-art source-finding methods, PyBDSF and ProFound. ContinUNet was then tested on the MeerKAT International GHz Tiered Extragalactic Exploration Early Science data without retraining and was able to extract point-like and extended sources with equal ease; processing a 1.6 deg$^2$ field in $\lt $13 s on a supercomputer and $\approx$2 min on a personal laptop. We were able to associate components of extended sources without manual intervention with the powerful inference capabilities learnt within the network, making ContinUNet a promising tool for enabling science in the upcoming SKA era.

We present the first measurement of HI mass of star-forming galaxies in different large scale structure environments from a blind survey at $z\sim 0.37$. In particular, we carry out a spectral line stacking analysis considering $2875$ spectra of colour-selected star-forming galaxies undetected in HI at $0.23 < z < 0.49$ in the COSMOS field, extracted from the MIGHTEE-HI Early Science datacubes, acquired with the MeerKAT radio telescope. We stack galaxies belonging to different subsamples depending on three different definitions of large scale structure environment: local galaxy overdensity, position inside the host dark matter halo (central, satellite, or isolated), and cosmic web type (field, filament, or knot). We first stack the full star-forming galaxy sample and find a robust HI detection yielding an average galaxy HI mass of $M_{\rm HI}=(8.12\pm 0.75)\times 10^9\, {\rm M}_\odot$ at $\sim 11.8\sigma$. Next, we investigate the different subsamples finding a negligible difference in $M_{\rm HI}$ as a function of the galaxy overdensity. We report an HI excess compared to the full sample in satellite galaxies ($M_{\rm HI}=(11.31\pm1.22)\times 10^9$, at $\sim 10.2 \sigma$) and in filaments ($M_{\rm HI}=(11.62\pm 0.90)\times 10^9$. Conversely, we report non-detections for the central and knot galaxies subsamples, which appear to be HI-deficient. We find the same qualitative results also when stacking in units of HI fraction ($f_{\rm HI}$). We conclude that the HI amount in star-forming galaxies at the studied redshifts correlates with the large scale structure environment.

In this paper, we combine the Early Science radio continuum data from the MeerKAT International GHz Tiered Extragalactic Exploration (MIGHTEE) Survey, with optical and near-infrared data and release the cross-matched catalogues. The radio data used in this work covers 0.86deg(2) of the COSMOS field, reaches a thermal noise of 1.7 mu Jy beam(-1) and contains 6102 radio components. We visually inspect and cross-match the radio sample with optical and near-infrared data from the Hyper Suprime-Cam (HSC) and UltraVISTA surveys. This allows the properties of active galactic nuclei and star-forming populations of galaxies to be probed out to z approximate to 5. Additionally, we use the likelihood ratio method to automatically cross-match the radio and optical catalogues and compare this to the visually cross-matched catalogue. We find that 94 per cent of our radio source catalogue can be matched with this method, with a reliability of 95 per cent. We proceed to show that visual classification will still remain an essential process for the cross-matching of complex and extended radio sources. In the near future, the MIGHTEE survey will be expanded in area to cover a total of similar to 20 deg(2); thus the combination of automated and visual identification will be critical. We compare the redshift distribution of SFG and AGN to the SKADS and T-RECS simulations and find more AGN than predicted at z similar to 1.

We present the discovery of the most distant OH megamaser to be observed in the main lines, using data from the MeerKAT International Giga-Hertz Tiered Extragalactic Exploration (MIGHTEE) survey. At a newly measured redshift of $z = 0.7092$, the system has strong emission in both the 1665MHz ($L \approx 2500$ L$_{\odot}$) and 1667 MHz ($L \approx 4.5\times10^4$ L$_{\odot}$) transitions, with both narrow and broad components. We interpret the broad line as a high-velocity-dispersion component of the 1667 MHz transition, with velocity $v \sim 330$km s$^{-1}$ with respect to the systemic velocity. The host galaxy has a stellar mass of $M_{\star} = 2.95 \times 10^{10}$ M$_{\odot}$ and a star-formation rate of SFR = 371 M$_{\odot}$yr$^{-1}$, placing it $\sim 1.5$dex above the main sequence for star-forming galaxies at this redshift, and can be classified as an ultra-luminous infrared galaxy. Alongside the optical imaging data, which exhibits evidence for a tidal tail, this suggests that the OH megamaser arises from a system that is currently undergoing a merger, which is stimulating star formation and providing the necessary conditions for pumping the OH molecule to saturation. The OHM is likely to be lensed, with a magnification factor of $\sim 2.5$, and perhaps more if the maser emitting region is compact and suitably offset relative to the centroid of its host galaxy's optical light. This discovery demonstrates that spectral line mapping with the new generation of radio interferometers may provide important information on the cosmic merger history of galaxies.

We present a new machine learning (ML)-driven source-finding tool for next-generation radio surveys that performs fast source extraction on a range of source morphologies at large dynamic ranges with minimal parameter tuning and post-processing. The construction of the Square Kilometre Array (SKA) radio telescope will revolutionize the field of radio astronomy. However, accurate and automated source-finding techniques are required to reach SKA science goals. We have developed a novel source-finding method, ContinUNet, powered by an ML segmentation algorithm, U-Net, that has proven highly effective and efficient when tested on SKA precursor data sets. Our model was trained and tested on simulated radio continuum data from SKA Science Data Challenge 1 and proved comparable with the state-of-the-art source-finding methods, PyBDSF and ProFound. ContinUNet was then tested on the MeerKAT International GHz Tiered Extragalactic Exploration Early Science data without retraining and was able to extract point-like and extended sources with equal ease; processing a 1.6 deg(2 )field in <13 s on a supercomputer and approximate to 2 min on a personal laptop. We were able to associate components of extended sources without manual intervention with the powerful inference capabilities learnt within the network, making ContinUNet a promising tool for enabling science in the upcoming SKA era.

The interplay between atomic gas, the star-formation history of a galaxy and its environment are intrinsically linked, and we need to decouple these dependencies to understand their role in galaxy formation and evolution. In this paper, we analyse the star formation histories (SFHs) of 187 galaxies from the MIGHTEE-HI Survey Early Science Release data, focusing on the relationships between HI properties and star formation. A strong correlation emerges between a galaxy's HI-to-stellar mass ratio and the time of formation, alongside an inverse correlation between stellar mass and time of formation, regardless of the inferred SFH. Additionally, galaxies with lower stellar masses and higher HI-to-stellar mass ratios exhibit longer gas depletion times compared to more massive galaxies, which appear to have depleted their gas and formed stars more efficiently. This suggests that smaller, gas-rich galaxies have higher depletion times due to shallower potential wells and less efficient star formation. Furthermore, we explore the connection between spin-filament alignment and HI content. We find no significant correlation between peak star formation activity and proximity to filaments. However, we do find that the two galaxies in our sample within 1 Mpc of a filament have very low gas-depletion timescales and have their spin axis misaligned with the filament, suggestive of a link between the galaxy properties and proximity to a filament.

ABSTRACT The transformation and evolution of a galaxy is strongly influenced by interactions with its environment. Neutral hydrogen (H i) is an excellent way to trace these interactions. Here, we present H i observations of the spiral galaxy NGC 895, which was previously thought to be isolated. High-sensitivity H i observations from the MeerKAT large survey project MIGHTEE reveal possible interaction features, such as extended spiral arms and the two newly discovered H i companions, that drive us to change the narrative that it is an isolated galaxy. We combine these observations with deep optical images from the Hyper Suprime Camera to show an absence of tidal debris between NGC 895 and its companions. We do find an excess of light in the outer parts of the companion galaxy MGTH$\_$J022138.1-052631, which could be an indication of external perturbation and thus possible sign of interactions. Our analysis shows that NGC 895 is an actively star-forming galaxy with a SFR of 1.75 ± 0.09[M⊙/yr], a value typical for high-stellar mass galaxies on the star-forming main sequence. It is reasonable to state that different mechanisms may have contributed to the observed features in NGC 895, and this emphasizes the need to revisit the target with more detailed observations. Our work shows the high potential and synergy of using state-of-the-art data in both H i and optical to reveal a more complete picture of galaxy environments.